1. Field of the Invention
The present invention relates to an electromagnetic switch, and is desirably applicable to an electromagnetic switch for a starter of a vehicle.
2. Description of Related Art
A known electromagnetic switch for a starter, which has high durability for sliding and high protection against corrosion at a sliding surface between a movable core and a metal sleeve, is disclosed in Japanese Patent No. 2646893.
The prior art electromagnetic switch is characterized in that a metal film, which is softer than the outer peripheral surface of the metal sleeve and has a small potential difference with the metal sleeve, is formed on an outer periphery of the movable core.
Specifically, the soft metal film, which has Vickers hardness of Hv30 nd is made of tin (Sn), is formed on the outer peripheral surface of the movable core made of mild steel. The metal sleeve is made of brass or stainless steel. The movable core slides on the inner surface of the sleeve via the metal film. The high durability for sliding is obtained because the soft metal film functions as a lubrication layer. At the same time, the electrolytic corrosion is prevented even under submergence because the potential difference between the movable core and the metal sleeve is reduced by covering the movable core with tin.
However, when an electromagnetic switch is applied to a starter, it may be required to reduce in size to accommodate it in the engine compartment and is also required higher output. Furthermore, more electromagnetic force may be required to drive a pinion gear of a starter motor via a drive lever in addition to actuating the switch. Further, since some drivers may keep operating the starter motor for long period, Joule heat generated at a solenoid coil may raise the temperature of the electromagnetic switch considerably.
More specifically, the sliding surface between the movable core and the metal sleeve may reach 250.degree. C. In that case, heat resistance may not be sufficient because the melting point of tin is 212.degree. C.